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Homework Help: Speed of Water Coming out of Vertical Cylinder

  1. Jan 8, 2014 #1
    1. The problem statement, all variables and given/known data
    A vertical cylinder with diameter 9 cm has an outlet on its side with diameter 3cm, water is filled to height 50cm and the outlet is 5 cm above the bottom of the cylinder. the outlet is closed by a valve, calculate the force required to stop the cylinder from moving when the valve is opened.

    2. Relevant equations
    Bernoulli's Equation [too long to type!]

    3. The attempt at a solution

    I'm struggling as no pressures are provided, and applying Bernoulli's equation isn't straightforward. I'm not sure whether it's possible to interpret it as the fact that the top of the cylinder is open, as this would make the problem simpler. I would really appreciate some insight to this question :)

    thanks in advance
  2. jcsd
  3. Jan 8, 2014 #2
    How do you find pressure of a liquid at a given depth? (or rather, when the surface of the water is a certain height above where you're desired measurement point)
  4. Jan 8, 2014 #3


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    You have water in the cylinder. You are given the height of the water above the bottom of the cylinder and the height of the outlet above the bottom as well. Water is affected by gravity. You can determine the pressure of the water by a few simple calculations. Do you know what the term 'head' means in fluid statics?

    Rather than using Bernoulli's equation, how about Toricelli's Law:

  5. Jan 8, 2014 #4
    not sure how, as the pressure at the top of the cylinder is not given and the you can't use the equation P=P0+ρgh as the liquid is flowing
  6. Jan 8, 2014 #5


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    To get this cart moving, assume that the top of the cylinder is vented to atmosphere.
  7. Jan 8, 2014 #6
    ok, so using Toricelli's Law you can get mass flow rate, and therefore the momentum of the water coming out of the valve, and therefore the force needed to keep it still. Thank you for your help!
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